Sensitized photoconductive zinc oxide



United States Patent 3,507,649 SENSITIZED PHOTOCONDUCTIVE ZINC OXIDE LeeC. Hensley, 131 Mary St., Binghamton, N.Y. 13903 No Drawing. Filed Jan.31, 1967, Ser. No. 612,781 Int. Cl. G03g 5/00, 7/00 U.S. Cl. 96--1.7 10Claims ABSTRACT OF THE DISCLOSURE Dyestuff materials advantageouslyadapted for use as optical sensitizing agents with photoconductormaterials employed in the electrostatic printing operations.

The present invention relates in general to optically sensitizedphotoconductive layers comprising zinc oxide and in particular to theprovision of novel sensitizing dyestuffs for such purposes.

Electrostatic printing processes for the production of visible recordsor reproduction are well known in the art being extensively described inthe literature both patent and otherwise. In general, such processesencompass as the salient features of operation the conversion of alight-image or electrical signal into an electrostatic charge pattern onan electrically insulating layer. Image-forming development canthereafter be effected according to any one of several procedureswhereby to render the latent charge pattern visible. Electrophotographicprocesses based upon the utilization of photoconductive layers includeof course the xerographic methods in which an electrically conductivesupport is first subjected to a uniform electrostatic charge in the darkthis being accomplished for example by means of a high voltage coronadischarge whereby an electrostatic charge is created on the elementsurface. Latent image formation can thereafter be effected by focussinga light-image on the charged surface, the light energy serving toselectively dissipate the electric charge in proportion to the intensityof the incident light radiation, i.e., an imagewise dissipation of theelectric charge in accordance with the impressed lightimage. Theresidual charged areas of the photoconductive layers, i.e., thoseprotected by the image areas of the original and thus unaffected by theexposure radiation provide what is tantamount to a latent electrostaticimage pattern which can be readily rendered visible by applicationthereto of a suitable colorant, e.g., toner powder, having opticaldensity sufficient to permit visible discernment of the image areas andwhich readily adheres to the residual charged areas. By the foregoingelectrostatic development operation there is obtained a permanentvisible image which provides an exact replica of the original. It isappreciated of course that a number of ramifications to theaforedescribed basic process by way of improvement have been promulgatedin the art; invariably however, such methods depends for feasiblepractice upon the principal of light-induced charge-decay whereby toprovide the surface of the image-recording member with a residual chargepattern capable of conversion to a readily visually comprehensibleimage. In general, it is found to be more effective practice to transferthe developed image which for example, may be defined by a pigmentedresinous composition constituting a toner powder to a receiving sheet incontradistinction to methods wherein the photoconductive plate itselfprovides diice rect means for producing the desired photographic copyabsent any transfer operation involving a receiving or master sheet.Photoconductive layers for use in electrophotographic reproductionprocesses of the foregoing type are conventionally prepared with suchphotoconductor materials as selenium, cadmium sulfide, zinc oxide, etc.For a plurality of reasons, zinc oxide has proved to be particularlybeneficial for the vast majority of operations associated withelectrography. The zinc oxide photoconductor material is conventionallyemployed in photoconductive layers in the manner described, i.e., agrounded support usually paper, is initially rendered sensitive to lightby subjecting same to a blanket negative electrostatic charge on thezinc oxide layer in the substantial absence of any ultraviolet orvisible radiation. As previously mentioned this step can readily beeffected by means of ion transfer from a corona discharge. Followingexposure, the resulting latent image areas, i.e., nonlight-struckportions of the photoconductive layer are developed, for example, with apigmented resin powder having a charge opposite to the negative chargeof the unexposed areas of the photoconductive layer. In this fashion,the pigmented powder firmly attaches itself via electrical attraction tosuch negatively charged areas. The strength of adhesion of the resinpowder to the image bearing layer can be enhanced by a suitable fixingoperation as for example by simply heating the resinous material totemperatures sufficient to fuse or melt same whereby such resin becomespermanently affixed to the surface of the image layer. It is to beunderstood of course that the temperatures employed in this operationshould be selected so as to avoid any possibility of charring the papersupport. In any event, suitable methods for effecting the development ofa latent electrostatic image pattern are described in the prior art withrecourse to a particular one depending primarily upon the requirementsof the processor.

As previously mentioned, particularly beneficial results are obtainedwith the use of photoconductive layers containing as the photoconductorsubstance, zinc oxide. Despite the advantageous features inherent in theuse of this material, certain problems are nevertheless encountered asregards attempts to impart optimum spectral response thereto. Since theeffective photographic speed of the reproduction process vitally dependsupon the actinic response of the photoconductor material, the overridingimportance of this factor is readily evident. Thus, practically withoutexception, the commercial grade zinc oxide photoconductor materialsprovided specifically for use in connection with the formulation ofphotoconductive layers exhibit maximum or peak spectral response to buta rather limited region of the spectrum, primarily, the far blue and theultraviolet. In contradistinction, the vast majority of the lightsources customarily employed for electrophotographic exposures displaymaximum output in the visible spectral region, e.g., an ordinarytungsten light. Such restricted spectral response quite obviouslyimposes stringent and burdensome limitations upon the process in thoseinstances wherein zinc 0X- ide is employed as the photoconductormaterial since the corresponding requirement is presented that a lightsource having the proper radiant emission be employed.

In view of the premier commercial importance of zinc oxide in theelectrophotographic industry, considerable industrial activity hascentered around the research and development of various means by whichto extend the spectral response of zinc oxide photoconductors whereby toimpart thereto peak sensitivity in those spectral regions forming thelocus of the emissions characteriz ing those light sources which wouldordinarily be employed. At this point it should be mentioned that onesuggested remedy to the foregoing problem involves the use ofphotoconductive materials having a spectral response in the visiblespectrum. Photoconductor substances which have heretofore been suggestedfor such purposes include, for example, the colored oxides, sulfides,selenides, tellurides, and iodides of such materials as cadmium,mercury, antimony, bismuth, thallium, molybdenuln, aluminum, lead, zinc,etc. Although providing some measure of improvement, procedures basedupon the use of the latter photoconductor materials have neverthelessproved somewhat unsuitable for certain applications. Thus, for the mostpart, the industrial effort thus far expended has been concerned withthe development of materials capable of absorbing radiant energy and oftransferring the energy so absorbed to the photoconductor. Thus, it hasbeen suggested to incorporate sensitizing dyes with the zinc oxidephotoconductor for the purposes of imparting the requisite spectralsensitivity to the reproduction system. Representative dyestuffmaterials heretofore promulgated in this regard include, for example,rose bengal, eosin, malachite green, crystal violet, methylene blue,methylene grey, fluorescein and the like. Although the use of suchdyestuffs has contributed greatly to resolving the problems associatedwith zinc oxide spectral sensitivity, other problems of a rathersignificant nature have nevertheless arisen as an incident thereto.Perhaps the primary objection to the sensitizing dyestuffs thus farsuggested relates to their pronounced tendency to impart to thesensitizing formulation a spurious off-white tint or coloration thusvitiating to a significant extent attempts to achieve satisfactorycontrast, gamma and the like. More specifically, such dyestuffs lead tothe formation of tints which may be blue, green, yellow, orange or redas well as various shades and hues thereof. Moreover, as will beappreciated, the undesired coloration of the zinc oxide layer isobjectionable from an aesthetic standpoint the latter being a relativelyimportant consideration bearing directly upon the possibilities ofcommercial acceptance. In some instances, the recording element itselfmay be contemplated for further exposure sequence, e.g., the productionof either black and white or color prints therefrom. The deleteriouseffects directly attributable to any spurious tint or off-white shadepresent in the image bearing layer will, practically without exception,be manifested in the form of inferior photographic quality in theresulting print. Such adverse effects are particularly evident withregard to color reproduction since the presence of spurious colorationin the recording element gives rise to faulty absorption densities,i.e., any fugitive color density will effectively modulate the exposureradiation and thus to this extent, effect undesired shifts in the colorcomposition, color balance, etc., of the color print.

A further serious objection to a considerable number of the sensitizingdyestuffs thus far proposed concerns rather their instability undervarying conditions of pH. This imposes rather severe limitations on theprocessors latitude of operations tending to circumscribe severely therange of selection of many of the remaining ingredients to be employedin the coating formulation. For example, the pH hypersensitivity of manyknown sensitizing dyestuffs restricts the processor, for example, inregard to the nature of the resin materials which may be efficaciouslyemployed. For example, if the resin material selected is not correlatedwith the pH sensitivity characteristics of the sensitizing dye,undesired colorations develop in the coated layer and invariably becomeexceedingly more pronounced on standing and .4 thus are readily visuallyperceptible. Apparently, discoloration of the coated element resultsfrom the inter-layer diffusion of acidic or alkaline materials, as thecase may be, such conditions being conducive to the creation ofspurious, off-white tints. As explained hereinbefore, fugitivecoloration of the electrophotographic element is not only aestheticallydispleasing but, and perhaps more importantly, renders such elementsubstantially unsuitable for further photocopying operations. Thus, as aconcomitant to the use of sensitizing dystuffs possessed of the pHsensitivity property, it becomes incumbent upon the formulator to adjustor otherwise modify the coating composition by way of compensating forundesired shifts in coloration which would otherwise occur. In manyinstances, resort to the use of masking dyestuffs, i.e., dyestuffshaving spectral absorption substantially complementary to that of thefugitive color tint, is made mandatory. As will be appreciated, remedialtechniques of this nature can prove burdensome to the formulator,requiring relatively precise and predetermined adjustments in coatingformulae. The costs involved in implementing such techniques may Well beprohibitive.

Other problems of equal significance which have been noted to attend theuse of many of the sensitizing dyestuffs thus far provided relate totheir sub-optimum compatibility with one or more of the ingredientscomprising the electrostatic layer coating composition. In this regard,it is imperative to quality reproduction that the photoconductivecoating composition containing the zinc oxide photoconductor beprovided, prior to actual coating, in the form of a homogeneous anduniform dispersion of the involved ingredients. Any departure in thisconection from optimum uniformity of dispersion render the final coatingsubstantially incapable of uniform spectral response, i.e., the densityequivalent of a given exposure product will in all likelihood varythroughout the coated layer. Thus, the possibility that the recordingsystem will necessarily reflect the point-to-point density variations inthe original to be reporduced in substantially emasculated.

Thus, in accordance with the discovery forming the basis of the presentinvention it has been determined, surprisingly, that a relativelylimited class of dyestuff materials provide eminently suitablesensitizing agents for electrophotographic layer compositions, enablingthe attainment of high quality reproduction absent the undesirablefeatures characterizing sensitizing dyestuffs heretofore provided forsuch purposes.

Thus, a primary object of the present invention relates to the provisionof novel dyestuff materials advantageously adapted for use withelectrographic layer compositions and wherein the foregoing and relateddisadvantages are eliminated or at least mitigated to a substantialextent.

A further object of the present invention relates to the provision ofoptically sensitized electrophotographic layer compositions havingexcellent sensitometric properties, e.g., speed, contrast, gamma, ets.,said compositions being capable of yielding high qaulity reproduction.

Another object of the present invention relates to the provision ofoptically sensitized electrophotographic layers having excellent actinicresponse and stability characteristics, said layers being substantiallydevoid of any tendency to develop spurious coloration.

Other objects and advantages of the present invention will become moreapparent hereinafter as the description proceeds.

The attainment of the foregoing and related object is made possible inaccordance with the present invention which in its broader aspectsincludes the provision of sensitizing dyestuffs having excellentsensitizing properties for zinc oxide photoconductive layers adapted forelectrophotography, said dyestuffs corresponding to the following in oneof the positions indicated, i.e., R, R and R The structural formula:term carboxyalkyl as used herein, is intended to connote a I --Y N Ra 2"R RI 'I CH=OH CHC o f l; 5 f

\ oon= on=o--on=on I i-R2 O: n-1 /qwherein R, R and R independentlyrepresent alkyl, e.gl, carboxy group in free acid form connected to thedyestufi' methyl, ethyl, propyl, butyl, isobutyl, etc.; aralkyl, e.g.,molecule by an alkylene bridge; thus, such term includes benzyl,fi-phenethyl; hydroxyalkyl, e.g., hydroxethyl; carboxymethyl,carboxyethyl, carboxy-n-propyl, etc. The alkoxyalkyl, e.g.,B-ethoxyethyl; carbalkoxyalkyl, e.g., improvements provided by thepresent invention have been carbomethoxymethyl, carboethoxymethyl,carboethoxyascertained to obtain to an optimum extent when such ethyl;acyloxyalkyl, e.g., [3acetoxyethyl, and the like, with group compriseseither carboxymethyl or carboxyethyl. the provision that at least one ofR, R and R represents As particular examples of dyestufi materialsfalling carboxyalkyl, e.g., carboxymethyl, carboxyethyl, etc.; R withinthe ambit of the above structural formula and and R represent hydrogen,alkyl, e.g., methyl, ethyl, etc.; found to provide exceptional advantageas sensitizing aryl, e.g., phenyl; aralkyl, e.g., benzyl, phenethyl,etc.; agents with photoconductive layers contemplated for usehydroxyalkyl, e.g., B-hydroxyethyl, etc.; m, n, p and q inelectrophotography, there may be mentioned the foleach represents apositive integer of from 1 to 3 inclusive; lowing: X represents an acidanion, e.g., chloride, bromide, thiocyanate alkyl sulfate, such as,methylsulfate, ethylsulfate, gl s gigii fjggigigagfi i gi fi etc.;arylsulfate, etc.; arylsulfates, such as benzenesulfo- I nates,p-toluene sulfonate, etc.; Y and Y" each represents3136F131z'benzoxazohdene)ethyhdene]4'oxo lodldethe non-methallic atomsnecessary to complete a 5 or 6- Thlazohmum, 3 iarboxxethyl 2(3'carboxyeth yl membered heterocycle, and Y represents a memberChloroZ'benzothlazolyhdenemethyl)5'[%'(516'd1' selected from the groupconsisting of oxygen, sulfur, $222 13- yl--benzoxazolylidene)ethyl1dene14-0xo l Selemum and mtmgen' Thiazolinium,2-(3-carboxyethylbenzoselenazolylidene- Typical representatives of theheterocyclic nuclei inmeth yl)5-[2-(3-carboethoxymethyl-4-methylth1azolyl1- cluded Within the ambit ofthe definitlons for Y, Y and Y dene)ethylidene] 4 OXO iodide givenabovelmclgde for thllazole f f zfig Thiazolinium,3-carboxymethyl-2-(3-carboxymethyl-5- awe? :3- dialkyl indolenine,pyridine and the like. The aforemen-$222111:'ethylbenzoxazolyhdene)ethyhdeneM'om tioned heterocyclic nucleicmay further contain one or Thrazolmium,2-(3-carboxymethyI-Z-benzothiazolylidenemore groups which compriseconventlonal substrtuents with respect to dyestuffs of this generaltype. Such sub- 40 gig g g s 5 (3 methylbenzothlazolyhdene)4 l g iffitgl gxi 2 222 5 3 3 gf ig fi' g figiffigfi Thiazolinium,2(3-carboXymethyl-2-benzothiazolylidenee g chloro"iodo f alkoxye gmethoxsl ethox; etc mgtgyl)-3-ethyl-5-(3-methylbenzothrazolylidene)4-oxoJ 7 '7 7 '7. 7 a 3 i e EZSJSZJZZZZZYFLZZiZaififliiii'ilitifi fiii;specifically their relationship to dyestutf molecules of theigigzicggilglenemethyl)5'(3'methylthlazohnyhdene) type enumerated. Thesalient requirement with respect to such substituents is that they beessentially innocuous or izg i i fi s gig iifiifi i 332 2 non-reactiveand exhibit no tendency to deleteriously afp H dege) 4 OXO iodidp y e y1 feet the sensitizing properties of the dyestufl? molecule. As yspecific examples of heterocyclic nuclei falling within the One of theparticularly surprising discoveries of the definition of Y and Y" givenabove there may be menpresent invention concerns the fact that dyestuffsof the tioned the following: thiazole, 4-methylthiazole, S-methyltypedescribed above not only exhibit an exceptionally thiazole, 4phenylthiazole, 4,5 dimethylthiazole, benzohigh order of sensitizingefficiency, i.e., impart a high thiazole, 5,6 dimethylbenzothiazole, 4chlorobenzoorder of spectral response to zinc oxidephotoconductorthiazole, 4 -methylbenzothiazole, 5 -bromobenzothiazole,containing photoconductive layers, but in addition, are 5,6diphenylbenzothiazole, 6 bromobenzothiazole, 5- highly stable overvarying conditions of pH thus remethoxybenzothiazole, 6iodobenzothiazole, 4 ethoxymoving any limitation as regards the natureof the resin benzothiazole, 5,6-dimethoxybenzothiazole, 5 hydroxybindermaterial employed. In addition, their compatibility benzothiazole, 4methoxy-thianaphtheno 7, 6', 4, 5- With the various other ingredientsconventionally emthiazole, 4 methyloxazole, 4 phenyloxazole, 4,5diployed in photoconductive coating compositions presents methyloxazole,benzoxazole, 5 chlorobenzoxazole, 5- significant advantage. Suchdyestuffs are further atypical Phenylbenzoxalole, dimethylbenzoXalole,Y' in that photoconductive elements containing same may be be l a 6ChlofobeIlZOXaZOle, 5 hydroxybenzoxa' stored for extended periods oftime either before or after Z016, 4 methylselenalole, 4PhenylselenaZole, 13611205616 electrophotographic processing in thevirtual absence of nazole, 5 chlorobenzoselenazole, 5methoxybenzoselediscoloration Whether due to pH or other conditionsnazole, 5 hydroxybenzoselenazole, a-naphthoselenazole,

fl naphthoselenazole, thiazoline, 4 methylthiazoline, gig ggi sg fifig lig gf zi g g g i 33; -hl 'l',6-

qumohne 5 methylqumohne 8 C Oroqumo me 5 to about 75 milligrams perpound of 21110 oxide with ethoxyquinoline, 8 hydroxyquinoline,isoquinoline, 3,4- dihydroisoquinoline, 3,3 dimethylindolenine,pyridine, a range of from about 18 to about 24 mlnlgrams belng 4methylpyridine, 3 5 dimothylpyridino, 4 ohloropwp particularlypreferred. Such proportions are not critical di 3 hydroxypyridine, 4pheny1pyridine, and the 1i per se merely encompassing those values foundto assure It is of critical importance that the trinuclear complex theObtelltion of optimum results- The requirements of merocyanine dyederivatives of the present invention a particular process may welldictate the propriety of contain at least one N-bonded carboxyalkylgroup, i.e., departures therefrom. It will also be understood that suchdyestuffs may be employed singly or in admixture comprising 2 or moredepending primarily upon the requirements of the processor e.g., thepeak sensitivity values desired in the photoconductive layer.

The photoconductive layer compositions of the present invention may beprepared according to conventional procedures described in the priorart, i.e. utilizing conventional solvents, coating aids, driers, etc.,such ingredients being of an optional nature. In any event, theessential components of the photoconductive composition comprise thesensitizing dye, the photoconductor material, i.e., zinc oxide, thelatter being dispersed in an insulating binder material havingrelatively high dielectric strength and good electrical insulatingproperties. As particular examples of film-forming insulating bindersfound to be suitable for use herein there may be mentioned thefollowing: styrene-butadiene copolymers; silicone resins; soya-alkydresins; poly(viny1 chloride); poly(vinylidene chloride); vinylidenechloride, acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate,vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinylbutyral), polyacrylic and methacrylic esters, such as poly(methylmethacrylate), poly(n-butylmethacrylate), poly(isobutyl methacrylate),etc.; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylenepolymers; polyesters, such as poly(ethylenealkaryloxyalkyleneterephthalate); phenol-formaldehyde resins; ketone resins; polyamide;polycarbonates; etc. Methods of making resins of this type have beendescribed in the prior art, for example, styrene alkyd resins can beprepared according to the method described in US. Patents 2,361,019 and2,258,423. Suitable resins of the type contemplated for use in thephotoconductive layers of this invention are sold under such trade namesas Vitel PE-lX, Cymac, Piccopale 100, and Saran F-220. Other types ofbinders which can be used in the photoconductive layers of the inventioninclude such materials as paraffin, mineral waxes, etc. Other polymericmaterials found to be especially advantageous include, for example, apolyester material available commercially from the Celanese Corporationof America under the commercial trade name designation Epitex 1311,which is prepared by reacting epichlorohydrin with bisphenol A employingthe former in slight molar excess and thereafter reacting the polyetherobtained with a mixture of a dimerized fatty acid and soya fatty acid.The resultant product comprises a linear, acetone-soluble, nonheatcurable polymer material containing epoxy groups. Methods for thepreparation of such polymers are described, for example, in U.S.P.2,970,983. A further material found to be admirably suited for useherein comprises a product available commercially from the PennsylvaniaIndustrial Chemical Company under the trade name designation Piccolasticwhich is identified as being a low molecular weight (on the order ofapproximately 400), low melting point (approximately 75 C.) polystyreneresin. In accordance with the present invention, it has been ascertainedto be especially advantageous to employ the resinous polymeric materialsin admixtures comprising 2 or more and thus to capitalize on thesuperior properties which may typify specific ones. The term resinousbinder as used herein is thus to be accorded a significance consonanttherewith, i.e., extending to either the singular or conjuctive use ofsuch resin materials.

Suitable solvents for effecting homogeneous dispersion of theingredients comprising the layer composition include, for example,toluene, Xylene, benzene, acetone, 2-butanone, chlorinated hydrocarbon,e.g., methylene chloride, ethylene chloride, etc. ethers, e.g.,tetrahydrofuran or mixtures of such solvent materials.

Alternatively, the ingredients comprising the photoconductive coatingcomposition may be provided in the form of an aqueous system incontradistinction to an organic solvent system. Improved sensitizationresults with either method. Again, recourse to either a solvent oraqueous system will be dictated in large part by the requirements of theprocessor.

Application of the photoconductive coating composition to the supportmaterial can be effected according to standardized methods, well knownin the prior art. Thus, for example, coating methods such asdoctor-blade, swirling, dip-coating and the like may be employed. Thethickness to which the photoconductive layer composition is depositedmay vary over a relatively wide range; in general, however, wet coatingthicknesses Within the range from about .001" to about 0.01" are foundto be eminently suitable for accomplishing the purposes of the presentinvention. Particularly beneficial results are found to obtain with theuse of wet-coating thicknesses falling within the range from about .002"to about .006". The support material employed may be any of theconventional materials promulgated in the art for the fabrication ofelectrostatic recording elements the principal requirement being thatsuch materials exhibit adequate electrical conductivity. Such materialsinclude, for example, paper (at a relative humidity above about 20%);aluminum-paper laminates; metal foils, such as, aluminum foil, zincfoil, etc., metal plates, such as, aluminum, copper, zinc, brass, andgalvanized plates; regenerated cellulose and cellulose derivatives;certain organic polymeric plastic materials, e.g., polyester andespecially polyesters provided with a thin electroconductive layer, suchas cuprous iodide coated thereon. Suitable supporting materials includein addition the humidity-independent conducting layers ofsemi-conductors dispersed in polymeric binders.

Other ingredients which may be incorporated into the coating compositionfor purposes of expediting the coating operation as well as to renderthe ultimate coating more suitable for use in the image recordingprocess include, for example, plasticizers; e.g., polymeric hydrocarbonshaving a fair degree of aromaticity and low iodine value; drying agents,e.g., cobalt naphthenate, manganese naphthenate and the like.

The zinc oxide photoconductor materials contemplated for use herein areavailable commercially. Desirably, the zinc oxide should be provided inthe form of relatively small particles having a mean diameter of lessthan about 0.5 micron. Particularly preferred for use herein is the zincoxide product produced according to the French Process such as FrenchProcess, Florence Green Seal, pigment grade zinc oxide commerciallyavailable from the New Jersey Zinc Sales Company Inc. of New York. Otherzinc oxide materials preferred for use herein include, for example, theproduct commercially known as St. Joe PC321 zinc oxide. Optimumrealization of the advantages provided by the present invention isobtained by employing the insulating binder in amounts sufficient toinsulate each of the zinc oxide particles from the remaining ingredientsof the coating composition. Such proportions can be readily determinedby rather route laboratory investigation.

The recording elements described herein can be advantageously employedwith any of the well known electrophotographic processes based upon theuse of photoconductive layers, e.g., the Xerographic process the latterbeing carried out by initially subjecting the electrophotographicelement to a blanket electrostatic charge, e.g., by the use of a coronadischarge. In view of the insulating character of the photoconductivelayer, attributable to the presence of the insulating resin bindermaterial, the uniform charge extent over the surface of thephotoconductive layer is retained, such layer also having the propertyof negligible conductivity in the dark or, as more commonly stated, highdark resistivity. Exposure of the photoconductive layer to light ervesto effect an imagewise dissipation of the electrostatic charge from thesurface of the layer thus leading to the formation of a latentelectrostatic charge pattern. The exposure may be effected through anegative by conventional exposure methods as for example, by contactprinting techniques or alternatively by lens projection of an image. Theextent of pointto-point charge dissipation depends correspondingly uponpoint-to-point intensity of the exposure illuminant. The residual chargepattern is thereafter rendered visible or otherwise developed bytreatmnt with a suitable colorant, pigment, etc. comprisingelectrostatic particles having a charge opposite to that of the residualcharge constituting the electrostatic latent image pattern, saiddeveloping agent being capable of ready visual comprehension. Thedeveloper agent may comprise, for example, a liquid developer in whichthe developing particles are suspended in an electrically insulatingliquid carrier. Developing methods of this type are of course well knownbeing described, for example, in U.S.P. 2,296,691 and in AustralianPatent 212,315. Other developing methods depending upon, for example,heat fusion of resin particles, image transfer are likewise well knownin the art and may be utilized to advantage in the practice of thepresent invention.

The following examples are given for purposes of illustration only andare not to be considered as necessarily limiting the present invention.

EXAMPLE I An electrophotoconductive coating composition is prepared inthe following manner:

To a solution consisting of: Toluene250 ml.

Cobalt naphthenate0.2l gm. Manganese naphthenateO.21 gm. Xylene 6 ml.

is added, With stirring, 151 gm. of Epitex 1311 (a polyester obtained byreacting epichlorohydrin with bisphenol A to form a polyether andthereafter reacting the latter with a mixture of dimerized fatty acidand soya fatty acid as described in U.S.P. 2,970,983). Upon completionof the Epitex addition, 454 gm. of zinc oxide photoconductor (St. JoeP0321) is added to the solution while stiirring. With stirring beingcontinued, a resin solution containing 35 gm. of piccolastic A75(polystyrene resin having a molecular weight of approximately 400 and amelting point of approximately 75 C.) is added. The medium is thereafterstirred and milled until smooth. At this point there is added 20 mg. ofthe sensitizing dyestuff,

Thiazolinium, 3 carboxymethyl 2 (3 carboxyethyl-S- methoxy 2benzoselenazolylidene methyl)5 [2- (3 benzyl- 2-benzoxazolylidene)ethylidene]4 oxo, iodide which has the followingstructural formula:

p, CCHCH=C 0 J3 o-on=o +1- -OCH3 N \N N CH2 i omooon cum coor-rdissolved in 20 ml. of methanol. The medium is thereafter stored for aperiod of approximately 30 minutes and then coated on Riegel 45 lb.conductive paper to a dried coating thickness of 20 lb. per 3000 sq. ft.

A coated paper is thereafter evaluated electrophotographically byexposure in a Bruning Copytron 2000 the latter comprising commerciallyavailable eletcrophotographic copying apparatus based upon dry tonerdevelopment. The prints obtained are characterized by excellent density,contrast, etc. and are totally devoid of spurious coloration thusproviding high contrast copy.

Moreover such prints exhibited no tendency to discolor upon standing forextending periods of time under varying conditions of heat, humidity,etc. In addition, the coated paper could be stored prior toelectrophotographic process, exhibiting excellent stability upon agingfor protracted time periods despite relatively severe conditions ofheat, humidity, etc.

Upon examination of the various prints specimen it was readily evidentthat uniform dispersion of the sensitizing dyestuffs throughout thecoating composition had been achieved in view of the prevailinguniformity of density, contrast, etc. The same observations were notedin connection with coated paper samples which had been subjected toextensive aging prior to electrophotographic processing.

The dyestutf employed in the above example is prepared in the followingmanner:

A solution comprising 0.8 part of methyl-p-toluene sulfonate and 0.42part of the following compound:

(compound A) is heated at 131 C. for 20 minutes and allowed to cool.Thereupon, 0.3 part of the following compound:

N CH2 l COOH is added with 5 parts of methanol and 30 drops oftriethylamine. The solution is then heated on a steam bath and glacialacetic acid is added, whereupon the initial stages of dye formation isdetected. Thereupon, toluene is added as well as potassium iodide inacetone. The solution is then transferred to a centrifuge tube,centrifuged, decanted and the residue boiled out with toluene,recentrifuged (hot) and again decanted. The residue is boiled out withmethanol, cooled and filtered to yield 0.2 part of dyestutf which, uponanalysis, was determined to be:

Thiazolinium, 3 -carboxymethyl-2- 3-carboxyethyl-5-methoxy-2-benzoselenazolylidene-methyl) 5 [2- 3-benzyl-Z-benzoxazolylidene)ethylidene] 4-oxo, iodide having thestructural formula depicted, exhibiting a peak spectral absorption inmethanol at 572 Ill 1..

Compound A above is prepared by reacting 3-carboxymethyl rhodanine, inthe presence of methanol and triethylamine with the following compound:

1 1 EXAMPLE 11 The procedure described in Example I is repeated exceptthat the dyestuif employed comprises the following:

Thiazolinium, 3-carboxyethyl-2- 3-carboxyethyl-5-chloro-2-benzothiazolylidene-methyl)5 [2 (5,6-dimethyl-3-ethyl2-benzoxazolylidene)ethylidene] 4-oxo, bromide having the followingstructural formula:

The above dyestuff which exhibits a peak spectral absorption in methanolat 586 mg is prepared in a manner identical with that described inExample I.

EXAMPLE III Example I is repeated except that the dyestuff employedcomprises the following:

Thiazolinium, 2-(3-carboxyethylbenzoselenazolylidenemethyl 5- [2-3-carboethoxymethyl-4-methylthiazolylidene)ethylidene] 4-oxo, iodide Thedyestuif employed in Example III is prepared in the following manner:

A solution comprising 0.8 part of methyl-p-toluene sulfonate and 0.37part of the following compound:

is heated at a temperature of 131 C. for 25 minutes and allowed to cool.Thereupon 0.38 part of the followin compound is added:

I COOH along with 4 parts of methanol and 30 drops of triethylamine. Thesolution is stirred, and warmed on a steam bath for 3 minutes whereupon30 drops of acetic acid and 30 parts of toluene are added. The solutionis allowed to stand for a brief time whereupon dye precipitation iseffected by the addition of ether. The dye precipitates in the form ofan oily substance which is thereafter decanted, triturated with ether,boiled out with toluene, decanted and boiled out twice with methanol.There is obtained 0.2 part of a dyestuff which upon analysis wasdetermined to be:

Thiazolinium, 2-(3-carboxyethylbenzoselenazolylidenemethyl 5- [2-3-carboethoxymethyl-4-methylthiazolylidene)ethylidene] 4-oxo, iodidehaving the structural formula previously illustrated and exhibiting apeak spectral absorption in methanol at 612 m In each of Examples II andIII the prints obtained exhibited the superior properties described inExample I. Significantly, it is found by way of comparison thatsensitizing dyestuffs of the type described in the prior art whensubjected to the identical processing, yield prints of markedly reduceddensity and contrast. In fact, in order to achieve photographic speed,contrast, comparable to those typifying the use of the sensitizingdyestuffs of the present invention it was necessary to increase by aconsiderable margin the quantity of sensitizing dyestuff employed. Thus,one of the salient advantages of the present invention is at onceapparent, namely synergistic sensitizing results can be obtained despitethe use of the sensitiving dyestuif in reduced amounts. This effects acorrelative mitigation in problems associated with spurious color tintsarising from the use of the sensitizing dyestuffs in exaggeratedquantities, i.e., on the order of those required for effective use withprior art sensitizers.

Similar results are obtained when the procedures exemplified arerepeated but employing as the sensitizing dyestuff, the following:

Thiazolinium, 3-carboxymethyl-2 (S-charboxymethyl-S-chloro-2-benzothiazolylidene-methyl)5[2,5,6-dimethyl-3-ethylbenzoxazolyidine)ethylidene] 4-oxo bromide.

Thiazolinium, 2-(3-carboxymethyl-2-benzothiazolylidenemethyl)-3-ethyl 5(3-methylbenzothiazolylidene)4- oxo iodide.

Thiazolinium, 2-(3 carboxymethyl 2 benzothiazolylidenemethyl)-3-ethyl 5(2-[ethyl-2-benzothiazolylidene] ethylidene 4-oxo iodide.

Thiazolinium, 3-carboxyethyl-2-(4,5 diphenyl-3-ethyl-2-oxoazolylidenemethyl)5 (3 methylthiazolinylidene) 4-oxo iodide.

Thiazolinium, I i-carboxymethyl 2 (3-ethyl-5-methyl- 4-phenyl 2thiazolylidenemethyl)5-(3 methylthiazoliny1idene)4-oxo iodide.

The sensitizing dyestuffs of the present invention may be employed toadvantage either singly or in admixture. Moreover, they may be used incombination with one or more of the dyestufl sensitizing materialsheretofore described in the art. In any event, it is recommendedpractice to maintain the proportions of the instant dyestufi productswithin the concentration range hereinbefore specified in order to assurethe obtention of optimum results. It is envisaged that in some instancessensitizer dyestuff concentration may be considered desirable which aresubstantially in excess of the delineated range. For example, it may bethat the speed requirements of the process may dictate such a departure.In such instances it is advisable to include one or more additionaldyestuffs in the photoconductive coating composition which performprimarily a masking function, i.e., to the substantial exclusion of asensitizing function in order to assure the suppression of any possiblefugitive tint which might otherwise develop. It will be furtherunderstood that such masking dyestuffs may be included as optionalingredients despite the use of the sensitizing dyestuffs of the presentinvention within the preferred concentration range; however, for thevast majority of commercial applications their use would hardly berequired.

The present invention has been disclosed with respect to certainpreferred embodiments thereof, and there will be obvious to personsskilled in the art modifications, equivalents or variations thereofwhich are intended to be included within the spirit and scope of thisinvention.

wherein R, R and R independently represent a member selected from thegroup consisting of alkyl, aralkyl, hydroxyalkyl, alkoxyalkyl,carbalkoxyalkyl, acyloxyalkyl with the provision that at least one of R,R and R represent ca'boxyalkyl; R and R represent a member selected fromthe group consisting of hydrogen, alkyl, aryl, aralkyl, hydroxyalkyl, m,n, p and q each represents a positive integer of from 1 to 3 inclusive,Y and Y" each represents the atoms necessary to complete a 5 or6-membered heterocycle, Y represents a member selected from the groupconsisting of oxygen, sulfur, selenium and nitrogen, and X represents anacid anion.

2. A composition according to claim 1 wherein said sensitizing dyestuficomprises thiazolinium, B-carboxymethyl-Z-(3-carboxyethyl-5-methoxy2-benzoselenazoly1- idene-methyl)5-[2-(3-benzyl 2benzoxazolidene)ethylidene]-oxo iodide.

3. A composition according to claim 1 wherein said sensitizing dyestuffcomprises thiazolinium, 3-carboxyethyl-2-(3-carboxyethyl 5 chloro2-benzothiazo1ylidene-rnethyl(5-[2-(5,6-dimethyl3-ethyl-2-benzoxazolylidene)ethylidene]4-oxo bromide.

4. A composition according to claim 1 wherein said sensitizing dyestuifcomprises thiazolinium, 2-(3-carboxyethylbenzoselenazolylidene-methyl)5[2 (Ii-carboethoxymethyl-4-methylthiazo1y1idene)ethylidene]4 oxo iodide.

5. A composition according to claim 1 wherein said sensitizing dyestulfcomprises thiazolinium, S-carboxymethyl-2-(3-carboxymethyl 5chloro-Z-benzothiazolyl- 14 idene-methyl)5-[2 ,5,'6-dimethy1 3ethylbenzoxazolylidene)ethylidene]4-oxo bromide.

6. A composition according to claim 1 wherein said sensitizing dyestuffcomprises thiazolinium, 2-(3-carboxymethyl-2 benzothiazolylidenemethyl)3 thyl-S-(3- methylbenzothiazolylidene) 4-oxo iodide.

7. A composition according to claim 1 wherein said sensitizing dyestufl?comprises thiazolinium, 2-(3-carboxymethyl-2-benzothiazolylidenemethyl)3ethyl 5-(2-[3- ethyl-2-benzothiazolylidene]ethylidene)4-oxo iodide.

8. A composition according to claim 1 wherein said sensitizing dyestuffcomprises thiazolinium, 3-carboxyethyl-2-(4,5-diphenyl 3ethyl-2-oxazolylidenemethyl) 5-(3-methylthiazolinylidene)4-oxo iodide.

9. A composition according to claim 1 wherein said sensitizing dyestuifcomprises thiazolinium, 3-carboxymethyl-2-(3-ethyl 5methyl-4-phenyl-2-thiazolylidenemethyl) 5-(3-methylthiazolinylidene)4-oXo iodide.

10. An electrophotographic recording element comprising a backing memberovercoated with the composition of claim 1.

References Cited UNITED STATES PATENTS 3,047,384 7/1962 Jones et al.961.7 3,288,610 11/1966 Gotze 961.7 X

FOREIGN PATENTS 292,828 11/ 1953 Switzerland. 541,245 9/ 1955 Belgium.

GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant ExaminerUS. Cl. X.R.

" UNITED STATES PATENT OFFICE 6) CERTIFICATE OF CORRECTION Patent No,Dated 2]., Invenmfl Lee C. Hensley It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

In the title, Column 1, line 3, for "Lee C. Hensley, 131 Mary St.,Binghamton, N.Y. 13903", insert,

--Lee C. Hensley, Binghamton, N.Y., assignor to GAF Corporation, NewYork, N.Y., a corporation of Delaware-- SIGNED AN SEALER o'cT 271970SEAL Attest:

Edward M. Fletcher, 11-. v F R-1AM E. 'summm,

oma

Attcsting Officer sion of t nts

